The materials known as dental ionomer cements have many applications in dentistry including use as filling materials for restoring teeth and for cementing inlays and crowns into place in the tooth, providing a base or a lining in a tooth cavity, and as adhesives or sealants. An ionomer cement is formed by reacting 1) an ionomer polymer with 2) a reactive glass. This reaction is typically done in water.
Ionomer polymers traditionally have been copolymers of two or more monomers. For example, itaconic acid and acrylic acid have often been copolymerized to form such an ionomer polymer. The polymer is formed by reacting the two monomers together using a free radical polymerization mechanism. The ionomer polymer is called such because of its inherent acidity resulting from the numerous acid pendant groups.
After the polymer is made it is often dissolved in water for later mixing with the reactive glass. The reactive glass used in the ionomer cement is often an ion-leachable glass, such as those based on calcium aluminosilicate glasses, or more recently, borate glasses. For a general discussion, see Prosser et al., Polyelectrolyte Cements, Wilson and Prosser, eds., Developments in Ionic Polymers-1, Chapter 5, Applied Science Publishers (London and New York, 1983). The glass is finely ground into a powder to facilitate mixing with the ionomer polymer solution.
In the setting reaction, the glass powder behaves like a base and reacts with the acidic polyelectrolyte, i.e., ionomer polymer, releasing metal ions to form a metal polysalt which acts as the binding matrix. Water serves as the reaction medium and allows the transport of ions in what is essentially an ionic reaction. The setting reaction is therefore characterized as a chemical cure system that proceeds automatically upon mixing the ionomer polymer and glass powder in the presence of water. The cements set to a gel-like state within a few minutes and rapidly harden to develop strength. See, e.g., Prosser et al., J. Chem. Tech. Biotechnol., 29, 69-87 (1979).
Chelating agents, such as tartaric acid, have been described as useful for modifying the rate of setting, e.g., to provide longer working times for the cements. See, e.g., U.S. Pat. Nos. 4,089,830, 4,209,434, 4,317,681, 4,374,936, and 4,758,612. Longer working times afford the dentist more time to mix the cement and apply the cement to the tooth. Unfortunately, when working times are lengthened by the usual methods, setting times are generally also lengthened. A longer setting time decreases the efficiency of the dentist and increases the amount of time the patient must spend in the dental chair. The role of tartaric acid has been explained as involving the temporary withholding of cations from crosslinking the polyanion chains (i.e., the ionomer) through complex formation. See generally, Prosser et al., Polyelectrolyte Cements, supra at Chapter 5.
Many commercially available glass ionomer cements include such chelating agents, and as a result are characterized by working times that are on the order of 1 to 2 minutes, but relatively long setting times, e.g., on the order of 4 to 15 minutes. During this set time the cement must be protected from being washed away by moisture from the mouth (e.g., through the use of cotton pads) but also must not be allowed to dry out. Such conditions can lead to discomfort for the patient as well as the added burden of having to spend extra time in the dentist's chair. Thus present day glass ionomer cements, although beneficial clinically, are quite technique-sensitive, as well as time-consuming for the dentist and patient.
Recently a photocurable ionomer cement has become available commercially. This cement system can provide a long working time and can be cured on demand by exposure to an appropriate source of radiant energy. This cement system is described in European Patent Application No. 0 323 120 and is commercially available as Vitrebond.TM. Light Cure Glass Ionomer Liner/Base (3M).
The final set cement must be both durable against frictional wear and resistant to degradation by an aqueous environment. Unfortunately, current ionomer cement systems frequently do not adequately resist degradation when exposed to water and may over time be undesirably eroded away.